Not true at all. First and foremost, "the whole number of the House has long been viewed as the number of Members elected, sworn, and living". So if they murdered all of the Democrats the remaining Republicans would have enough members to maintain a quorum.

https://crsreports.congress.gov/product/pdf/RL/98-988

But there are other ways to get around this as well. Specifically, read sections:

- Quorum in the Case of Catastrophic Circumstances

- Voting and Quorum Procedures in the House of Representatives

All Dems murdered would be catastrophic circumstances and would require the 72 hour window with the failure to reach quorum report. That’s plenty of time for states to re-appoint all of the senators and prevent the take over of the upper house.